An electric discharge machining apparatus melts and removes a work in a machining liquid by supplying pulse field power between an electrode and the work each provided in the machining liquid. It is a known fact that powder due to machining is generated in a machining gap in electric discharge machining and unless the powder due to machining is removed with any method, insulation of and repetition of electric discharge in the machining gap can not be maintained in a normal state, which causes an arc discharge state, whereby the electric discharge machining is badly affected so that machining efficiency is lowered and a condition of a machined surface is degraded.
A method of removing the powder due to machining includes methods such as machining liquid injection, blowing, and suction, and so-called a jumping operation in which an electrode is reciprocated intermittently and at high velocity is well known as one of the methods used in addition to the above methods.
In a case of machining any work having a form to which liquid machining such as injection, blowing, or sucking for discharging the powder can not physically be applied, an electrode jumping operation is the only method for removing the powder, and for this reason this operation is generally regraded as one of applicable conditions therefor.
FIG. 5 is a view showing an example of the jumping operation. In FIG. 5, the sign J indicates Z-axial movement characteristics of an electrode (jumping path characteristics) and the sign P indicates electric discharge machining time characteristics in a jumping operation. It should be noted that the sign Pp indicates a machining period while the sign P1 indicates a non-machining period (idling time).
Jumping conditions include a jumping height (a jumping-up rate) Jup, a jumping time Jt, and a jump-down time (machining time) Jdown, and selection of those parameters (jumping conditions) is an extremely important factor for efficiently discharging out the powder and improving a machining velocity.
For example, a jumping-up rate Jup raised to a sufficiently high level enables discharge of machining powder from a deep machining hole in a case where a depth of machining becomes deeper. A jumping velocity Jup/Jt is a factor that gives effects over reduction of time in jumping (non-machining time P1) which does not contribute to machining as well as over the powder discharge efficiency based on the pumping effects of jumping, and also which gives large effects over a machining velocity.
As described above, a machining time, a machining velocity, and a maximum machining depth are affected depending on the way of setting a jumping-up rate, a jumping time, a jump-down time, and a jumping velocity, each of which is a parameter for a jumping operation, namely depending on the way of setting a path to be made by the jumping operation, and for this reason an operation for setting of conditions for jumping operations is very important.
Generally, by setting a jumping velocity and a jumping-up rate to possibly maximum ones respectively, the powder discharge efficiency becomes higher, and for this reason the machining efficiency and machining velocity are improved.
As shown in FIG. 5, however, a machining gap is widened in jumping up so that a discharge pulse is not generated and machining does not proceed, namely an idling time P1 is generated, and machining indicates time is equivalent to a machining period Pp which is not jumping up, so that a ratio of the actual machining time in the entire machining time is reduced when jumping height Jup becomes higher, which causes reduction of a machining velocity.
Namely, the jumping operation has a factor contradicting such that a machining velocity is improved by enhancing machining-liquid discharge efficiency as well as by increasing a ratio of a jump-down time (time in machining ) Jdown as much as possible in the entire machining time.
One of the methods for solving this contradiction is, as shown in FIG. 6, to reduce the time Jt required for jumping by making a jumping velocity V (V=Jup/Jt) as fast as possible, however, if a jumping velocity V is speeded up, a load to a mechanical structure of the electric discharge machining apparatus becomes larger, which causes bad influences such as induction of mechanical vibrations or reduction of mechanical precision. For this reason, there is sometimes a case where a jumping velocity can not easily be increased. Especially in a case where an area to be machined is wide, it is known that a negative pressure is generated in a machining gap due to a delay in sucking a machining liquid when an electrode is pulled up so that the electrode can not easily be held up.
A reaction force generated when an adjacent plane form (electrode) is pulled up from a liquid (machining liquid) is described by the expression (1). Herein, F is a machining reaction force generated by a jumping operation, k: a proportionality constant, S: a machining area, V: a jumping velocity, and D: a machining gap. EQU F=k.multidot.S.sup.2 .multidot.(V/D.sup.3) (1)
Namely, force generated in a machining gap in an initial stage of the jumping operation is proportional to the square of a machining area S and is inversely proportional to the cube of a machining gap D. For this reason, the machining reaction force F becomes extremely large in machining of a large area, especially in an operation for finishing.
Generally, when a large area is to be machined, as the machining reaction force F according to the expression (1) is proportional to the jumping velocity V, the jumping velocity V is set to a lower value so as not to induce reduction of precision due to an excessive load applied to a machine. It is understood from the expression (1) that the machining reaction force F is proportional to the square of an area from the expression (1), if an area to be machined becomes twice, a jumping velocity V has to be reduced to 1/4 thereof. For this reason a time required for jumping largely increases, which causes a machining velocity to be reduced.
FIG. 7 shows jumping operations in a case where the jumping velocity is reduced. When the jumping velocity is reduced, a proportion of a period of time in machining Pp in the entire machining period of time is reduced, and also the machining velocity is reduced by the same proportion.
To avoid the phenomenon described above, there is sometimes employed a method of accelerating a jumping velocity by using the fact that a machining gap D is widened and also machining reaction force F is reduced by jumping. Namely, the jumping velocity v is changed so that the value (V/D.sup.3) in the expression (1) is constant.
With this method, the ratio of a machining area S does not always lead to increase in a jumping time V as it is, and for this reason a time required for jumping can further be optimized.
FIG. 8 is a view showing operations according to the method, and a leading edge section Jacc as well as a falling edge section Jred each of the jumping operations are accelerated or decelerated according to a machining gap length. In this case, the machining gap during jumping down is shown as Ddown.
FIG. 9 shows the characteristics in a case where the machining gap Ddown during jumping down becomes narrower in the same method. When the machining gap D in the expression (1) becomes smaller like Ddown, accelerated/decelerated velocities in the leading edge section Jacc as well as in the falling edge section Jred each in the jumping operations are made further smaller. Even in a case where an area to be machined S is large, a load to the machine can be suppressed also by changing the accelerated/decelerated velocities.
It should be noted that a machining velocity in a path for acceleration or deceleration is theoretically obtained in proportion to the cube of the machining gap D, however, the same effects can be obtained by making approximations to other high-speed computable path in consideration of computing performance inside of a numerical control unit.
However, it is still required to decide a constant for a jumping velocity V according to an area to be machined S, so that it is necessary to reset a jumping velocity V according to the area to be machined S.
Generally, this change is one of elements in machining conditions, so that it is selectable as one of the machining conditions. Namely, a jumping velocity notch is selected according to the machining contents, but it is not always easy to accurately estimate a machining reaction force F or a load to the machine according to a machining area S as well as to a machining gap length (machining gap D), and to select an optimal jumping velocity notch. Especially, when a large area is to be machined, it is not easy to optimize the jumping condition and to increase the machining velocity.